For decades neutrinos have been failing to appear in detectors where they should be. Physicists think it's because the nearly massless particles "oscillate" into harder-to-detect varieties, or flavors, and have long sought ironclad evidence of the oscillations. Within the past few years, they have found such evidence for neutrinos from two of their three main sources: the sun and the atmosphere. Now, physicists have added the third source by showing that electron antineutrinos produced by nuclear reactors change type as they travel through Earth.
Scientists have known since the 1950s that they weren't seeing all the neutrinos coming from the sun. But they only nailed down the case for solar neutrino oscillation in 2001, when Canada's Sudbury Neutrino Observatory spotted a deficit of electron neutrinos from the sun together with a matching surplus of muon and tau neutrinos (ScienceNOW, 18 June 2001). It was clear that electron neutrinos were turning into the harder-to-detect muon and tau types. With neutrinos created in the atmosphere, the story was similar: There were too few muon neutrinos compared with electron neutrinos, but it took years to make a solid case for oscillations. In 1998, the Super-Kamiokande detector in Japan showed that the proportion of muon to electron neutrinos varied smoothly depending on how far the neutrinos traveled, a clear indication that the muon neutrinos were changing flavors as they move, altering the mixture of muon and electron neutrinos.
The story has now repeated itself with antineutrinos created in reactors. In 2002, the KamLAND collaboration, a group of scientists in Japan and the United States, used a large sphere filled with scintillating fluid buried underneath mountains near Toyama, Japan, to spot a shortfall of the particles (ScienceNOW, 6 December 2002). Now, in a paper just accepted by Physical Review Letters, the KamLAND group reports that sorting 258 neutrino collisions by energy yielded the distribution that oscillation would produce.
"It's strong evidence that it's [the] oscillations" that are responsible for the missing neutrinos, says Kevin Lesko, a collaborator at Lawrence Berkeley National Laboratory in Berkeley, California. Janet Conrad, a physicist at Fermi National Accelerator Laboratory in Batavia, Illinois, agrees. "It's a very nice result," she says, adding that the results "significantly" narrow the possible relative masses of two flavors of neutrino--crucial information for characterizing the particle.